Paper i proceeding, 2007

To improve the energy efficiency in the pulp and paper process, thermal integration of the evaporation plant with other parts of the plant is an option. Evaporation of black liquor up to higher dry solid content gives a higher heating value of the black liquor used in the recovery boiler. A high dry solid content means that the viscosity of the black liquor becomes very high. The link between viscosity and heat transfer for black liquor is therefore interesting. Experimental and theoretical knowledge is essential for proper design of evaporators for this purpose. A simplified model for the black liquor falling-film evaporation heat transfer coefficient is suggested. Without a substantial increase in uncertainty, only dependence on mass flow rate and viscosity is included in the model. The model is created from experimental data from experiments performed with a research evaporation plant with a 4.5 m long evaporator tube. Heat transfer in falling-film evaporators has been studied by other researchers before. The falling-film black liquor evaporation heat transfer coefficient dependence on mass flow rate (in a limited mass flow rate range) and viscosity according to this study is in agreement with some of these earlier studies. Above a certain mass flow rate limit, however, the experimental heat transfer coefficient was constant or even decreased with increasing mass flow rate.